(a) Field of the Invention
The present invention relates to a power supply, a display device, and a light source driving apparatus.
(b) Description of the Related Art
Display devices used for monitors of computers and television sets include self-emitting displays such as light emitting diodes (LEDs), electroluminescence (EL) devices, vacuum fluorescent displays (VFDs), field emission displays (FEDs), and plasma panel displays (PDPs), and non-emitting displays such as liquid crystal displays (LCDS) requiring a light source.
An LCD includes two panels provided with field-generating electrodes and a liquid crystal (LC) layer with dielectric anisotropy interposed therebetween. The field-generating electrodes supplied with electric voltages generate an electric field in the liquid crystal layer, and the transmittance of light passing through the panels varies depending on the strength of the applied field, which can be controlled by the applied voltages. Accordingly, desired images are obtained by adjusting the applied voltages.
The light may be emitted from a light source equipped in the LCD or may be natural light.
A lighting device for an LCD, i.e., a backlight assembly, usually includes a plurality of fluorescent lamps such as external electrode fluorescent lamps (EEFLs) and cold cathode fluorescent lamps (CCFLs), and a plurality of light emitting diodes (LEDs) as light sources, which uniformly transmit the light to the entire front surface of the LC panels from the rear of the LC panels.
When using the fluorescent lamps, characteristics of elements of the display device is deteriorated due to large consumption power and heating of the fluorescent lamps.
In addition, the fluorescent lamps have a bar shape, such that they easily break on impact. Moreover, since temperatures of the lamps vary in accordance with positions thereof, making the luminance of the lamps vary, and decreasing the image quality of the LCD.
However, when using the LEDs, since each LED is a semiconductor device, the lifetime of the LED is long, the lighting speed of the LED is fast, and the power consumption is low. The LED is also resistant to impact damage and can be miniaturized more easily.
To realize the benefits, the LEDs are equipped on a monitor for middle or large sized LCDs such as for a computer or a television set, as well as on small LCDs such as in a mobile telephone for a light source.
The fluorescent lamps are activated by an AC (alternating current) voltage, but the LEDs are activated by a DC (direct current) voltage. Accordingly, for using the LEDs, the backlight assembly includes a power supply converting an AC voltage into a DC voltage and changing a magnitude of the DC voltage into a predetermined magnitude.
The power supply includes a DC-DC converter converting a magnitude of a DC voltage.
There are various types of power supply for controlling the DC-DC converter, including a voltage-controlling type, a current-controlling type, and a sensorless current-controlling type. The voltage-controlling type has advantages in a simple circuit design due to one feedback path and receives almost no influence with respect to noise. The current-controlling type turns a switching element of the DC-DC converter on or off by using a clock signal with a predetermined frequency, and has advantages in a simpler circuit design and in providing a gain bandwidth different than those of the voltage controlling type.
The sensorless current-controlling type observes an average current flowing through a switching element of the DC-DC converter by using an observer-based technique, and it turns the switching element on or off based on the average current. However, the sensorless current-controlling type can be influenced by variation of an input voltage, harmonic wave components contained in the input voltage and so on, which decreases the reliability of the DC-DC converter and a response speed thereof.